Hens Eject Sperm from Unwelcome Suitors

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Chickens have their own battle of the sexes, and scientists have
discovered a secret strategy used by hens to control who
fertilizes their eggs: After mating, hens can eject the sperm of
less desirable, low-status roosters.

A new study has shown that, during an average ejection, a hen
jettisons 80 percent of the sperm a rooster deposits in her
reproductive tract. This has a huge impact on
the competition among males fighting to father her future
chicks, according to study researcher Tommaso Pizzari, an
evolutionary biologist and university lecturer at Oxford
University in the United Kingdom.

"It is beginning to appear females can play a much more subtle,
but powerful role in the battle for fertilization," Pizzari told
LiveScience.com.

A few things to know about
chicken sex : Both sexes are promiscuous, mating with multiple
partners. Hens, however, often don't have a choice in mates. They
prefer males at the top of the pecking order, but other roosters
with lower status will force the hens — about half their size —
to mate. Rather than attempt to fight off the undesirable mates,
hens appear to have developed a more subtle way to reject them.

Scientists already knew that hens could eject sperm, but in the
recent study, they set out to find evidence that hens were
actively using this technique
to control fertilization.

Using chickens from a flock that lived in a semi-feral setting,
similar to their wild ancestors, the red jungle fowl, Pizzari and
other scientists led by Rebecca Dean who conducted the study
while at Oxford and is now at University of Uppsala in Sweden
mated hens with various roosters; the scientists ranked roosters'
social status from 1 to 6, with 1 being the most dominant. They
then videotaped any sperm ejection that followed the mating and
collected the results. To determine how this compared with the
total sperm the roosters had deposited, the researchers captured
all of their semen during another set of controlled mating
attempts.

Their results confirmed that sperm from the least desirable,
low-status roosters suffered the most for several reasons.

When mating with a series of roosters, hens ejected more semen
from the later mates. Since lower-status roosters don't get the
first shot at the hens, for this reason alone, their sperm are
more likely to be ejected, Pizzari explained. But even
controlling for mating order, status had a strong effect on whose
sperm the hens kept. In addition, lower status roosters were more
likely to ejaculate more semen in one shot, and the team found
that hens were more likely to eject larger ejaculations.
[ Longest
Known Sperm Create Paradox of Nature ]

"It is likely in more natural situations, subdominant males are
disadvantaged in many ways," Pizzari said.

The study appears in the September 2011 issue of the journal The
American Naturalist.